1. Field of the Invention
The present invention relates to a hydraulic circuit control device that selectively supplies oil to two oil passages with an oil pump.
Priority is claimed on Japanese Patent Application No. 2005-102507, filed Mar. 31, 2005, and Japanese Patent Application No. 2005-336782, filed Nov. 22, 2005, the contents of which are incorporated herein by reference.
2. Description of Related Art
A vehicle drive system has been developed in which either the front or rear wheels are powered by a main drive source such as an internal combustion engine, while an auxiliary drive source is provided by an electric motor for the other wheels.
Under normal driving conditions in a vehicle equipped with such a drive system, the main drive source drives the front or rear wheels, while the auxiliary drive source is activated to transmit drive power to the other wheels when, for example, setting off in adverse road conditions. In such a vehicle, a hydraulic engaging/disengaging device such as a hydraulic clutch is provided in the power transmission mechanism to deliver power from the driving electric motor serving as the auxiliary drive source, with such a clutch being suitably controllable in accordance with the vehicle running state. For example, at times when driving or regeneration of the electric motor is not required, by using the clutch to cut off power transmission with the electric motor, drive power loss arising from co-rotation of the electric motor can be reduced.
A control unit for a hydraulic actuator resembling the engaging/disengaging device described above has been proposed that provides an accumulator in the oil passage leading to the actuator to reduce the power loss of the oil pump (see, for example, Japanese Unexamined Patent Application, Publication No. 2003-54279).
In this control unit for an actuator, the accumulator is installed in the oil passage on the actuator side, and a check valve that only allows inflow of oil to the actuator side is interposed between the oil pump and the actuator, with the oil pump operating only when the pressure in the accumulator falls.
In the aforementioned control unit, the oil supplied from the oil pump is supplied only to the oil passage on the actuator side, which requires a high pressure. However, there is a clear need for shared use of the same oil pump for oil passages that require a low pressure and a high flow rate for lubrication and the like.
One solution that has been studied is to provide in the oil pump supply passage an oil passage switching valve that switches connection between the oil passage that requires a high pressure and the oil passage that requires a low pressure and a high flow rate, and operate the oil passage switching valve in accordance with requirements on the system side. In this case, it would be necessary to control the pump driving electric motor that drives the oil pump simultaneously with switching the oil passages in order to adjust the oil being supplied to a suitable oil pressure and flow rate.
However, in the case of always using speed control to control the pump driving electric motor, sudden fluctuations in the hydraulic load when used for the oil passage that requires a high pressure may cause step out of the electric motor.
Controlling the pump driving motor by torque control has also been investigated, but in this case, overspeed of the electric motor occurs when used for the oil passage that requires a low pressure and a high flow rate, leading to high power consumption, which is not desirable from the standpoint of energy conservation.